APPENDIX ADOE-HDBK-1017/1-93Properties of MetalsOrganics are easily permeated by tritium (gas or water) and are therefore subject to disruptionof their bulk chemistries. There are few or no mechanisms for rapidly delocalizing beta energy,and substantial mobility of organic chains occur within polymer structures (particularlyamorphous regions). Once formed, reactive organic intermediates can thus react with each other.These effects are important when considering the design of tritium systems. Damage tocomponents, such as gaskets, valve tips, and O-rings, must be carefully considered. Componentfailure during service can cause a major release of tritium. Because elastomer seals oftenbecome embrittled, maintenance on nearby sections of piping may cause seals to develop leaksas the result of mechanical movement in the seal area.Figure A-1 illustrates several polymerFigure A-1 Modifications to Polymer Chains Due to Irradiation chain modifications that take placefollowing activation by beta radiation toionic or excited species. Cross-linkingand degradation are the most importantprocesses to the mechanical properties ofthe polymer. These both compete in amaterial, but those polymers that aremost sterically hindered appear topreferentially degrade. Steric hindranceprevents neighboring chains from linkingand also imparts structural strains that arerelieved upon chain scissioning. Cross-linking is noted mechanically by anincrease in tensile strength and a decreasein elongation, whereas degradation isevidenced by a decrease in tensilestrength, an increase in elongation, andsoftening of the polymer to a gummyconsistency.Several factors effect polymer stability.First, energy-delocalizing aromaticstructural groups increase polymerstability by distributing energies ofexcited states. In addition, halogen atomswithin polymers generate free radicalsand thus promote radiation damage. MS-02Page A-6Rev. 0-A